1. Field of the Invention
The present invention relates to a brush contact type charging unit of an image forming apparatus. The brush contact type charging unit comprises a brush charger and a constant-current regulated power supply for the brush charger. The present invention particularly relates to the constant-current regulated power supply.
2. Description of the Related Art
An image forming apparatus is an apparatus such as an electrophotographic duplicating apparatus or printer. In the image forming apparatus, an electric image signal is converted to a toner image formed on an image forming medium and recorded on a recording sheet by transferring the toner image onto the recording sheet. In forming the toner image onto the surface of the image forming medium, the following process is performed: (1) the surface of the image forming medium is first charged uniformly by an electrostatic charging unit; (2) an optical signal obtained by converting the electric image signal is irradiated on the uniformly charged surface of the image forming medium thereby, producing a latent image on the image forming medium; and (3) the toner image is produced by developing the latent image.
In the above process, the electrostatic charging unit is very important for forming a high quality toner image onto the image forming medium. This is because, the quality of the toner image depends on the uniformity of the charge on the image forming medium.
There are three types of electrostatic charging units, namely corona discharging type, a roll contact type and a brush contact type. In the three types, however, the brush contact type charging unit, which will be simply called the "brush type charging unit" hereinafter, has come to be widely used. This is because the corona discharging type and roll contact type charging units have the following problems. The corona discharging type charging unit charges the image forming medium by performing corona discharging through an air gap, so that it requires a high voltage such as several thousands volts. Therefore, a large size and a high cost are required for the power supply. Furthermore, the usage of the high voltage produces ozone which shortens the life of the image forming medium. The roll contact type charging unit charges the image forming medium by using a roll made of an electroconductive elastic roll material attached to the surface of the image forming medium and rotated with the movement of the medium. The roll contact type charging unit also has a problem concerning uniformity of the charge on the image forming medium. Dust in the image forming apparatus is easily stuck to the roll material, producing dusty zones in places on the surface of the roll material, and the dusty zones are difficult to remove. Therefore, even though the roll contact type charging unit can use a low power supply voltage such as one thousand volts, the uniformity of the charge on the image forming medium is difficult to maintain because of the dusty zones. In the brush type charging unit, the dust problem does not occur. In the brush type charging unit, the image forming medium is charged by the brush charger consisting of a plurality of brush fibers arranged transversely and perpendicularly to the moving direction of the image forming medium, and attached to the surface of the image forming medium. The brush type charging unit charges the image forming medium under a low power supply voltage as in the roll contact type charging unit and has no problem of uniformity due to the dust as in the roll contact type charging unit.
Recently, the size of the image forming apparatus has become compact. Therefore, the size of the image forming medium also becomes small, so that a high charging speed is required between the image forming medium and the charging unit if the same desirable recording speed is kept. However, it becomes hard to keep good uniformity when the charging speed increases because the charging current at an initial period of transient charging is easily changed by an external cause. Accordingly, even in the brush type charging unit there is a problem of maintaining good uniformity of the charge on the image forming medium.
In the contact type charging unit such as the roll contact type or the brush contact type charging unit, the uniformity of the charge on the image forming medium strongly depends on the atmosphere, particularly relative humidity, around the charging unit, when a constant-voltage regulated power supply is used in the contact type charging unit. In the roll contact type charging unit, the dependence of the uniformity on the humidity has been studied in Japanese laid-open patent application TOKUKAISHO 56-132356 to Doi et al. of Sept. 12, 1986. According to Doi, less affect on uniformity by humidity is improved by applying a constant-current regulated power supply to the roll contact type charging unit instead of the usual constant-voltage regulated power supply. Doi teaches that the uniformity can be improved by applying the constant-current regulated power supply to the roll contact type charging unit. However Doi teaches nothing concerning the brush type charging unit. Incidentally, the problem of uniformity due to the dusty zones is not solved in Doi.
When the contact type charging unit uses the constant-current regulated power supply, the uniformity of the charge on the image forming medium is deteriorated by pin-holes in the image forming medium. Generally, the image forming medium is a photosensitive layer formed on a metal substrate by dipping the metal substrate into liquid photosensitive material. Therefore, from a view point of cost performance, it must be assumed that a few pin-holes will appear in the photosensitive layer because of very small air bubbles having been produced during the dipping. In the pin-hole, the metal substrate is exposed even though the size of the pin-hole is very small. Therefore, the problem of the uniformity of the charge due to the pin-holes usually occurs in the case of the contact type charging unit, particularly in the case of the brush type charging unit. In Doi, the problem due to the pin-holes is discussed; however the pin-holes discussed in Doi are quite different from those mentioned above. The pin-holes in Doi are those produced by electrical break down between the surface of the image forming medium and the roller due to a high electric field.
The constant-current regulated power supply has also been applied to the corona discharging type charging unit. This is disclosed in Japanese Patent publication 62-11345 to Suzuki et al. in Mar. 12, 1987. According to Suzuki, the uniformity of the charge on the image forming medium is also influenced by variation in humidity. This problem of the uniformity can be improved by applying the constant-current regulated power supply to the corona discharging type charging unit instead of the usual constant-voltage regulated power supply. However, Suzuki teaches nothing relating to the present invention because the constant-current regulated power supply in Suzuki is a very high voltage (several tens thousand volts) power supply and the charging mechanism of Suzuki is quite different from that of the present invention.